DrupalDateTime::format

public DrupalDateTime::format($format, $settings = array())

public function format($format, $settings = array()) {
  $langcode = !empty($settings['langcode']) ? $settings['langcode'] : $this->langcode;
  $value = '';
  // Format the date and catch errors.
  try {
    // Encode markers that should be translated. 'A' becomes
    // '\xEF\AA\xFF'. xEF and xFF are invalid UTF-8 sequences,
    // and we assume they are not in the input string.
    // Paired backslashes are isolated to prevent errors in
    // read-ahead evaluation. The read-ahead expression ensures that
    // A matches, but not \A.
    $format = preg_replace(array('/\\\\\\\\/', '/(?<!\\\\)([AaeDlMTF])/'), array("\xEF\\\\\\\\\xFF", "\xEF\\\\\$1\$1\xFF"), $format);

    // Call date_format().
    $format = parent::format($format, $settings);

    // Translates a formatted date string.
    $translation_callback = function($matches) use ($langcode) {
      $code = $matches[1];
      $string = $matches[2];
      if (!isset($this->formatTranslationCache[$langcode][$code][$string])) {
        $options = array('langcode' => $langcode);
        if ($code == 'F') {
          $options['context'] = 'Long month name';
        }

        if ($code == '') {
          $this->formatTranslationCache[$langcode][$code][$string] = $string;
        }
        else {
          $this->formatTranslationCache[$langcode][$code][$string] = $this->t($string, array(), $options);
        }
      }
      return $this->formatTranslationCache[$langcode][$code][$string];
    };

    // Translate the marked sequences.
    $value = preg_replace_callback('/\xEF([AaeDlMTF]?)(.*?)\xFF/', $translation_callback, $format);
  }
  catch (\Exception $e) {
    $this->errors[] = $e->getMessage();
  }
  return $value;
}